1. Field of the Invention
This invention relates to an operating force controlling device for use with a construction equipment such as a crane, and more particularly to an operating force controlling device for providing an operation reactive force corresponding to a load pressure to an operating lever in order for an operator to sense initiation of movement of a suspended cargo with a hand manually when the suspended cargo is to be lifted or lowered.
2. Description of the Invention
A crane is equipped with a winch drum for lifting or lowering a suspended cargo, and a hydraulic motor for driving the drum is connected to the drum as disclosed in Japanese Utility Model Laid-Open No. 55-14199. The crane has a valve mechanism for controlling rotation of the motor. The valve mechanism includes a pilot valve connected to be operated by an operating lever, and a pilot type directional control device connected between the motor and a fluid supply source. When the lever of the crane is operated in the lifting direction, a pilot pressure is outputted from the pilot valve, and the directional control valve is changed over to its lifting position by the pilot pressure so that pressurized fluid is supplied from the fluid supply source to the motor. Then, the pressure (load pressure) on the fluid inlet side of the motor increases gradually, and when the load pressure exceeds a pressure corresponding to the magnitude of the load (load of the suspended cargo), the motor is activated to start the drum in its lifting direction. After that, the motor is driven with the load pressure corresponding to the magnitude of the load of the suspended cargo to carry out a lifting operation of the suspended cargo. Accordingly, if a change in load pressure of the motor is discriminated, then initiation of movement of the suspended cargo will be discriminated.
The crane disclosed in Japanese Utility Model Laid-Open No. 55-14199 mentioned above includes an operating force controlling device for enabling an operator to sense such change in load pressure of the motor with a hand which is operating the lever. The operating force controlling device includes a pilot valve connected to be operated by a lever, and a pair of cylinders operatively connected to the pilot valve for providing an operation reactive force to the lever. If the lever is operated to the lifting side to change over the directional control valve to cause the motor to rotate in order to lift a suspended cargo, the load pressure of the motor is inputted to a chamber of one of the cylinders by way of a corresponding one of pilot pipe lines from pipe lines communicating with ports on the opposite sides of the motor to push up a piston of the pilot valve and a rod connected to the piston. The rod is contacted with a pivotal position portion connected to the lever to urge the lever to return to its neutral position. An operation reactive force thus acts upon the lever. The operation reactive force increases in proportion to the load pressure of the motor. Accordingly, when an operator operates the lever, the load pressure of the motor can be sensed by sensing the operation reactive force by way of the lever.
However, the operating force controlling device has such a structure that the load pressure of the motor upon lifting and lowering of a suspended cargo is inputted directly to the chambers of the cylinders, and particularly when the load of the suspended cargo is heavy and the load pressure of the motor is high, the high pressure fluid will flow into the chambers of the cylinders. Accordingly, seal portions and so forth of the cylinders are required to have a sufficiently high strength to bear a high pressure. Consequently, the device is high in cost.
Further, the operating force controlling device is constituted such that the diameter (pressure receiving area) of the piston on the lifting operation side is equal to the diameter (pressure receiving area) of the piston of the lowering operation side, and as the load pressure is inputted to one of the chambers behind the pistons, the operation reactive force is controlled linearly at a fixed rate in proportion to the load pressure. However, when the suspended cargo is lowered, the load pressure varies only a little after changing over of the directional control valve until a counterbalance valve interposed between the directional control valve and the motor is opened, and after the counterbalance valve is opened and the suspended cargo starts to move in the lowering direction, the load pressure becomes substantially fixed irrespective of the magnitude of the load of the suspended cargo. Accordingly, even if the load pressure of the motor upon lowering operation is inputted to the chambers of the cylinders in the operating force controlling device, since the amount of change of the load pressure is small, the amount of change of the operation reactive force is so small that it is difficult to manually sense such change and accordingly it is difficult to manually sense initiation of movement in the lowering direction of the suspended cargo. On the other hand, when the suspended cargo is lifted, particularly where the load of the suspended cargo is small and the motor has a low load (the motor is in a region wherein the load pressure is low), the amount of change of the operation reactive force is so small that it is difficult to manually sense a change of the load pressure as a change of the operation reactive force. Accordingly, it is difficult to manually sense initiation of movement of the suspended cargo.